Color flat panel displays, such as liquid crystal displays and the like, typically incorporate color filters used to provide pixels with color. One technique for fabricating color filters involves a laser-induced thermal transfer process. A particular prior art thermal transfer process is illustrated schematically in FIG. 1A. A substrate (often referred to as a “receiver element”) 10 is overlaid with a donor element (often referred to as a “donor sheet”) 12. In the case of color filter fabrication, substrate 10 is typically made of glass and has a generally planar shape. Donor element 12 is typically a sheet which is relatively thin and relatively flexible when compared to substrate 10. Donor element 12 may be made of plastic, for example. Donor element 12 incorporates donor material (not shown). The donor material may comprise a colorant, a pigment or the like used to fabricate the color filter.
Donor element 12 is image-wise exposed to selectively transfer donor material from donor element 12 to substrate 10. Some exposure methods involve using one or more controllable lasers 14 to provide one or more corresponding laser beams 16. In currently preferred techniques, laser beam(s) 16 induce the transfer of donor material from the imaged regions of donor element 12 to corresponding regions of substrate 10. Controllable laser(s) 14 may comprise diode laser(s) which are relatively easy to modulate, have relatively low cost and have relatively small size. Such laser(s) 14 are controllable to directly image-wise expose donor element 12. In some embodiments, masks (not shown) are used to image-wise expose donor element 12.
Once the donor material has been image-wise transferred from donor element 12 to substrate 10, it is typically necessary to remove the used (“spent”) donor element 12 from substrate 10. For example, during the fabrication of color filters, a first donor element 12 may be used to apply red colorant to substrate 10, a second donor element 12 may be used to apply green colorant and a third donor element 12 may be used to apply blue colorant. After use, each spent donor element 12 is removed from substrate 10 prior to application and use of a subsequent donor element 12.
In many cases, at the conclusion of the imaging process, the “transferred” donor material may adhere partially to substrate 10 but may also remain partially adhered to donor element 12. This partial adherence of the donor material to both substrate 10 and donor element 12 can make it difficult to remove donor element 12 from substrate 10.
In prior art techniques, donor element 12 is removed from substrate 10 using a roller 18 incorporating one or more suction features 20. Roller 18 is brought into proximity of edge 12A of donor element 12 (as shown by arrow 19) and then suction is applied through suction features 20, such that edge 12A of donor element 12 is secured to suction features 20. Roller 18 is then rotated (as shown by arrow 22) and translated (as shown by arrow 24) to wind donor element 12 off of substrate 10 and onto the circumferential surface 18A of roller 18 and to thereby peel donor element 12 from substrate 10.
This technique for removing donor element 12 from substrate 10 suffers from a number of drawbacks which tend to reduce the quality of the image on substrate 10 (i.e. the donor material imparted onto substrate 10). One of these drawbacks is shown in FIG. 1B. As donor element 12 is wound onto the cylindrical surface 18A of roller 18, there reaches a time where the portion of donor element 12 currently being peeled from substrate 10 approaches leading edge 12A of donor element 12 on cylindrical surface 18A. Donor element 12 has a non-negligible thickness, which causes the take-up of donor element 12 to exhibit a discontinuity 26 or the like in the portion of donor element 12 which overlaps leading edge 12A. This discontinuity 26 can effect the donor material transferred to substrate 10 or otherwise result in artefacts in the image imparted onto substrate 10. This phenomenon may be referred to as “print-through”. In some circumstances, such print-through artefacts can repeat at regular intervals related to the circumference of roller 18.
Another drawback associated with prior art techniques for removing donor element 12 from substrate 10 relates to variations in the peel angle θ (i.e. the angle at which donor element 12 is pulled from substrate 10—See FIG. 1B). Such variations in peel angle θ may be caused by the thickness of donor element 12, stretching of donor element 12, variations in the peeling tension or other factors. Variations in peel angle θ may also cause artefacts in the image imparted onto substrate 10.
There is a general desire to provide methods and apparatus for more effectively removing spent donor elements from a substrate after donor material has been transferred from a donor element to the substrate.